Ad extensions are extras that can be place on an advert on Google's Sponsored Ads. Below are the main examples of different ad extensions. They all increase the overall click through rate (CTR) of a campaign which results in lower cost per click costs.
The green box shows reviews and is available after 30 reviews on particular websites and a good average rating.
The red box contains Sitelinks - which link to individual pages of a website.
The blue box shows a Click to call feature or call extension and on Mobiles can be used to directly make a call.
The yellow box shows a Location extension and only shows to local computers.